1. Field of the Invention
The invention relates to a furnace for heating articles in which exhaust gases of the combustion chamber are recirculated for preheating of the articles, the furnace having a partially sealed pressure chamber disposed at the front end of the preheating section to prevent the introduction of ambient air into the furnace.
2. State of the Prior Art
It is common to use furnaces having an elongated heating chamber for heating billets for logs used in extrusion or similar metal working operations. A typical furnace has an elongated heating chamber in which burners are disposed along the length of the furnace to direct burning gases against the billets as they move through the heating chamber. Such furnaces are often of the flame impingement type in which the burner flames envelope the billets within the furnace.
It is known to use the exhaust gases of the combustion chamber to preheat the billets within the furnace. In U.S. Pat. No. 3,837,794, issued Sept. 24, 1974, a billet heating furnace similar to that described above is disclosed in which hot combustion gases are drawn through a preheating zone for heat exchange with the articles passing therethrough. It is also known to exchange the heat of the combustion gases with combustion air and supply this air to a preheating zone as well as to burners in a heating chamber. Such a furnace is disclosed in U.S. Pat. No. 4,022,571, issued May 10, 1977. Additionally, U.S. Pat. No. 3,841,614, issued Oct. 15, 1974, discloses an apparatus for preheating articles in which the hot exhaust gases are supplied through jet nozzles in a preheater zone in order to provide a turbulent flow about the articles being preheated.
In U.S. Pat. No. 4,309,171 (issued Jan. 5, 1985) there is disclosed and claimed a furnace of the above-described type wherein a pressure balance is maintained throughout the length of the furnace and a special pressure chamber is provided near the entrance end of the furnace with a pressure sealing arrangement to maintain the chamber at or slightly above atmospheric pressure, while the remainder of the preheater section is below the pressure in the pressure chamber. In this manner, the proper flow of gases through the furnace is maintained, while at the same time, the pressure at the entrance and exit ends of the furnace is maintained at or slightly above atmospheric pressure so as to minimize the introduction of ambient air into the furnace. The pressure chamber is maintained at an elevated pressure through special jet pipes which extend through the preheater section and into the special pressure chamber. It has been found that it is difficult to adjust the pressure in the pressure chamber with the jet pipes. It is desirable that the pressure in the pressure chamber be easily adjustable by external means so as to maintain the appropriate pressure in the pressure chamber.